In recent years, the processing time for the development of photographic photosensitive materials (referred to as "photosensitive materials" below) has been greatly shortened by advances in high-temperature rapid processing and in the automatic development processing of various photosensitive materials. To achieve rapid processing, developing solutions which produce satisfactory image density in a short time, and photosensitive materials with excellent developability and which provide adequate black densities and dry within a short time after washing are desired. A method which is effectively used to improve the drying properties of photosensitive materials comprises adding an appropriate amount of a film hardener (gelatin crosslinking agent) during the photosensitive material coating stage. The film hardener decreases the amount of water contained in the material prior to the start of drying by reducing the amount of swelling of the emulsion layer(s) and surface protective layer during the development, fixing and washing stages. Although the drying time is shortened in proportion to the amount of film hardener used, development is delayed, a lowering in sensitivity and soft gradation occur, and the covering power is diminished by the reduction in the amount of swelling. Furthermore, when carrying out high-temperature rapid processing using processing agents wherein the developing solution and fixing solution substantially have no gelatin hardening effect, as disclosed in JP-A-63-144084 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), it is essential to adequately harden the photosensitive material prior to processing, and, with silver halide emulsions having slow developability, it is not possible to effect processing in a short time. On the other hand, methods for increasing the development activity of the processing solutions are also known, namely the amounts of the developing agents and auxiliary development agents in the developing solution may be increased, the pH of the developing solution may be increased, or the processing temperature may be raised. However, with each of these methods, there are disadvantages such as a loss in the preservative properties of the processing solutions, or susceptibility to fog and soft gradation may occur, even if the sensitivity is increased.
Aside from considerations of rapid processing, further increases in sensitivity and improvements in the graininess of the photosensitive materials is desired.
If increased sensitivity is brought about by an increase of the grain size, then there is a deterioration in graininess.
Thus, there is little gain if higher sensitivities are not achieved at the same grain size (for tabular grains, the same grain size being the same projected surface area diameter and the same thickness) or if graininess is not improved at the same sensitivity.
The object of the present invention is to provide a silver halide photographic emulsion having a high covering power and which exhibits markedly superior developability, sensitivity/fogging ratios and graininess using tabular-grained emulsions without increasing the projected surface area diameter or thicknesses.
Techniques using tabular grains to improve factors as described above have been disclosed in U.S. Pat. Nos. 4,439,520, 4,425,425 and 4,414,304, but the present invention differs by providing a performance which clearly excels the effects of the abovementioned patents and it relates to a technique for optimally exploiting the performance of tabular grains.
Furthermore, a technique for controlling the development starting point by deploying silver halide on specific surface portions (for example, an apex) of the tabular grains (so-called epitaxial growth) has been disclosed in JP-A-58-108526, but such a technique is undesirable from the standpoint of its applicability to production, such emulsions having poor aging stability in solution or in storage. The present invention also improves these aspects.
Moreover, it is known, as disclosed in U.S. Pat. Nos. 3,628,969 and 4,225,666, that it is possible to carry out simultaneous spectral sensitization and chemical sensitization by adding sensitizing dyes at the same time as the chemical sensitizers or, disclosed in JP-A-58-113928, to carry out spectral sensitization prior to chemical sensitization, or to initiate spectral sensitization by addition of the spectral sensitizing agents before the completion of silver halide grain precipitation production. In addition, it is known in the art to add a portion of the spectral sensitizing compounds prior to chemical sensitization and the remaining portion thereof after chemical sensitization as taught in U.S. Pat. No. 4,225,666, and that the addition may be made at any time during silver halide grain formation, notably using the method taught in U.S. Pat. No. 4,183,756.
However, with these known techniques, the abovedescribed silver halide crystal production methods differ from that of the present invention with respect to, for example, the amount of the sensitizing dyes and the grain formation method employed, and the effects of these known production methods clearly do not extend as far as the effects obtained by the method of the present invention.
Japanese Patent Application No. 62-141112 and U.S. patent application Ser. No. 07/202,852 (filed Jun. 6, 1988) disclose a technique for preparing high-sensitivity silver halide emulsions having good developability by ensuring that development commences from the apexes of each grains during the development processing, by the introduction of sensitizing dyes or silver halide adsorbing substances during the chemical sensitization. However, said technique has the following disadvantages: if large amounts of sensitizing dyes are introduced during chemical sensitization, the blackening phenomenon deteriorates markedly when outside pressure is applied to the silver halide grains; and when the photosensitive material is bent or rubbed, fogging and abrasion occur due to this pressure. The present invention eliminates these defects of the references.
Again, practical usage difficulties are encountered when silver halide adsorbing substances other than sensitizing dyes are introduced during chemical sensitization. This brings about an inhibition of sensitizing dye adsorption when color sensitization is then attempted. Residual color then remains after development processing of the photosensitive materials when dyes with a strong adsorbing power with respect to the silver halides have been used.
The present invention solves the above-noted problems and provides a high-sensitivity silver halide photographic emulsion having outstanding developability and with little pressure blackening.